In internal combustion engines a heat exchanger can be used in order to cool the lubricating oil of the internal combustion engine. The heat exchanger includes usually a heat exchanging element and an inlet as well as an outlet for the lubricating oil as well as an inlet and an outlet for a cooling liquid. The heat exchanger is usually connected by the lubricating oil circulation to a liquid filter. The liquid filter can be arranged remote from the heat exchanger or can be directly integrated into the heat exchanger unit. The entire heat exchanger is connected by a flange to an engine block wherein the unfiltered heated raw oil is introduced from the engine block through an inlet first into the heat exchanger and is cooled therein and subsequently leaves the heat exchanger through an outlet. Subsequently, the oil can be supplied to the dirty side of the filter element and can be filtered by the filter element. Through the clean side of the filter element the filtered and cooled oil is returned into the oil circulation in the engine block. The heat exchanger can be arranged also at the clean side of the filter.
In order to prevent that in particular after a cold start of the engine at very low temperatures the oil flowing into the heat exchanger as a result of its greatly increased viscosity at low temperatures causes blockage and makes more difficult stationary flow through the heat exchanger, a bypass can be branched off, for example, from the inlet into the heat exchanger and extend to the outlet of the heat exchanger or, in case of an existing filter, it may connected directly to the dirty side of the filter. The bypass can be open independent of the operating state and throttled by a constriction. Other heat exchangers include a pressure relief valve that usually is in the closed position and therefore blocks the bypass. When the pressure surpasses a permissible limit, for example, at low temperatures as a result of a blocked heat exchanger, the pressure relief valve opens and the oil can flow out directly. In this way, a blocked heat exchanger does not block the entire liquid circulation. In this way, in particular an improved cold start behavior is achieved.
DE 102 45 005 A1 discloses a liquid filter heat exchanger unit in which a bimetal element, depending on the temperature of the liquid, controls the flow into the heat exchanger or through the bypass in that when a specific switching temperature is surpassed or undershot, it automatically switches between two switching positions: it opens the bypass below the switching temperature and closes it above the switching temperature.
A disadvantage of this solution is that it enables only a relatively minimal flow rate while a great flow resistance is present. Also, this solution is not capable of opening the bypass in case of overpressure at high operating temperatures.
DE 102 05 518 discloses a thermostat-controlled valve with integrated bimetal element in which the bimetal element opens the valve cone when a certain limit temperature is reached.
A disadvantage of this solution is that, in comparison to a pure pressure-control valve, it requires the bimetal element as an additional component. Moreover, it enables only a minimal switching travel.
U.S. Pat. No. 6,746,170 discloses an oil module for an internal combustion engine in which a bypass is controlled by a thermostat valve that comprises a spring of a shape memory material and a counteracting conventional spring. In this connection, the conventional spring acts as a restoring spring for the shape memory spring. In this connection, the spring made of shape memory material acts in the same direction as the liquid pressure at the dirty side, the conventional spring acts opposite to the liquid pressure and provides the force for opening the valve.
Based on this prior art knowledge, it is an object of the present invention to provide a heat exchanger that improves the cold start behavior of an internal combustion engine.
In particular, the present invention has the object to configure a heat exchanger with constructively simple measures and without controlling action from the exterior in such a way that at low temperatures and minimal liquid pressure a bypass is opened and with open bypass a higher volume flow through the bypass is enabled and the flow resistance is lowered.